Added unit designators to MediaResource fields
Made DataSource.preroll() non-async

This fixed a regression introduced on the retrofit branch that was
 causing all HTTPResponseElements returned from the DataSource to
 be dropped (and their data not being incorporated into the HTTP
 response header)
Added method descriptions to HTTPGetHandler methods
Fixed a unit conversion bug in HTTPMediaResourceHandler.get_resource_duration()

It was returning seconds instead of microseconds. Added multiplier.
Added HTTPResponseElement.to_string() definition/implementations for debugging